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Nuclear Materials and Energy

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Effect of heating temperature on tritium retention in stainless steel type316 L

M. Matsuyamaa,⁎, H. Zushib, K. Tokunagab, A. Kuzminc, K. Hanadab

aHydrogen Isotope Research Center, Organization for Promotion of Research, University of Toyama, Gofuku 3190, Toyama 930-8555, Japanb RIAM, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, JapancNational Institute for Fusion Science, Toki, Gifu 509-5292, Japan

A R T I C L E I N F O

Keywords:Tritium retentionDegassing temperatureTritium exposure temperatureBIXSSurface analysisSS316L

A B S T R A C T

Dependence of heating temperature on tritium retention behavior in stainless steel type 316 L (SS316L) has beenexamined about each process of degassing and tritium exposure. Two kinds of SS316L samples were employed:bare SS316L and plasma-exposed SS316L. The amount of tritium retained in surface layers of a sample wasnondestructively measured by β-ray-induced X-ray spectrometry, and changes in the surface state with heatingin vacuum were analyzed by X-ray photoelectron spectroscopy (XPS). Significant increase in tritium retention inbare SS316L samples appeared in a degassing temperature region above 600 K. Similar tendency of tritiumretention was observed for the plasma-exposed sample. It was seen that the degassing process prior to tritiumexposure significantly influenced to the tritium retention behavior. Furthermore, it was suggested from surfaceanalysis by XPS that chemical states of SS316L surface at high temperatures play an important role for tritiumretention behavior.

Introduction

From viewpoints of the safety and the economy of tritium in a futurefusion reactor, reduction of tritium inventory in the fusion devices isone of critical issues. For example, tritium inventory in theInternational Thermonuclear Experimental Reactor (ITER) is restrictedbelow 700 g as tritium retention [1]. The amount of tritium retained inplasma-facing materials (PFMs) such as divertor, first wall and otherplasma-facing components will be affected by not only plasma condi-tions and chemical properties of PFMs but also surface characteristics ofPFMs.

During a long operation of the fusion devices, surfaces of PFMs areirradiated with various high energy particles such as hydrogen isotopes,helium, and neutron. By the interactions between high energy particlesand PFMs, as well known, erosion and/or re-deposition of PFMs willtake place on the surface along with the radiation damages, and alsochemical and/or physical properties of the surface will change with anoperation time. Such changes should bring about significant impact onthe retention and release behavior of tritium.

Effects of re-deposition layers and radiation damages on tritiumretention have been mainly studied so far using deuterium [2–4], butfew examinations have used tritium except for data from TFTR and JETfusion facilities [5–6]. Matsuyama et al. have investigated tritium

retention behavior in stainless steel type 316 L (SS316L) samples ex-posed to plasmas in the Large Helical Device [7–9]. As a result, it wasclarified that the sample surface was covered with re-deposition layersconsisting of carbon, oxygen and metallic species such as Cr, Fe, and Ni.In addition, it was suggested that metallic species in the re-depositionlayers might play an important role as trapping sites of tritium, becausemetallic species are usually more active than the oxides and carbides fordissociation reaction of tritium molecules [10].

Surfaces of PFMs will expose to a high temperature under highvacuum condition as well as severe irradiation environment.Temperature and vacuum conditions of PFMs will affect chemicalproperties of the surfaces. Thus, effect of heating temperature on tri-tium retention was studied using SS316L as a model, and effect ofplasma exposure to SS316L on tritium retention was also examined.QUEST (Q-shu University Experiment with Steady-State SphericalTokamak) was utilized for plasma exposure of a sample, which is op-erated under high temperature wall (393 K). In addition, changes inchemical state of surface elements of SS316L with heating temperaturewere examined under high vacuum condition by using a technique ofsurface analysis.

https://doi.org/10.1016/j.nme.2018.05.024Received 21 October 2017; Received in revised form 29 March 2018; Accepted 29 May 2018

⁎ Corresponding author.E-mail address: toyama3h1949@gmail.com (M. Matsuyama).

Nuclear Materials and Energy 16 (2018) 52–59

2352-1791/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

T

Experimental

Thin plates of SS316L were used in this study as a sample (describedhereafter as “bare SS316L”), and the sample size was15×15×0.5 mm3. According to the material certificate, nominalatomic composition (at. %) of bare SS316L is as follows: Fe(65.3), Cr(19.0), Ni(11.9), Mn(1.89), Mo(1.28) and Si(0.72). All samples wereprovided for experiments after mechanically polishing by buff andrinsing with acetone.

As shown in the Fig. 1, four kinds of tritium exposure experimentsfrom No. 1 to No. 4 were conducted to study the effects of heatingtemperature on tritium retention in SS316L. At first, temperature de-pendence of tritium exposure was examined in the No. 1 experiment. Inthis run, tritium exposure temperature was changed from 393 to 623 K,where the degassing temperature prior to the tritium exposure proce-dure was fixed at 673 K in each run. On the other hand, in the No. 2experiment, the degassing temperature was changed from 393 to 673 K,where the tritium exposure temperature was kept constant at 393 K.Furthermore, effects of air exposure were examined in the No. 3 ex-periment. After degassing at 673 K, the sample was once exposed to anambient atmosphere for 2 h at room temperature. This is again degassedat 393 K and subsequently it was exposed to tritium gas at the sametemperature.

To examine effect of plasma exposure on tritium retention, in theNo. 4 experiment, one of bare SS316L samples was held near inner wallof QUEST by using a linear motion feed through which was connectedwith the MH10 port. It was located near the equator of QUEST. A bareSS316L sample was fixed by a sample holder attached at the top of thefeed through, and it was mainly exposed to hydrogen plasmas during aplasma campaign (ID No. 450-460) of the spring-summer season in2016. After plasma exposure, to expose to tritium gas, the plasma-ex-posed sample was transported from QUEST to Hydrogen IsotopeResearch Center (HRC), University of Toyama. The transport was car-ried out by enclosing the sample in a specially designed vacuum-tighttransport device.

Two tritium exposure devices (TED) are applicable in HRC: TED-1and TED-2. The former device is able to directly connect to the trans-port device mentioned above, and it is possible to move the sample into

an exposure part of TED-1 without air exposure. After transferring, thetritium exposure device was evacuated below 2×10−5 Pa at 393 K andthen tritium gas was introduced into TED-1 at the same temperature.This sample was repeatedly used for examination of effects of heatingtemperature. The latter one was mainly used for heating and tritiumexposure experiments.

All the tritium exposure tests were carried out for 4 h at a giventemperature and the total pressure of tritium gas was fixed at 1.3 kPa.Tritium gas was supplied by heating a tritium storage-supply materialincorporated into TED-1 or TED-2. The first tritium exposure in the No.4 experiment of the plasma-exposed sample was conducted by using atritium gas of about 0.5%-T in TED-1 and all of other tritium exposureexperiments were conducted using TED-2 in which tritium gas of 4.7%-T was stored. After tritium exposure, the residual tritium gas was re-covered by the tritium storage-supply device used to supply, and thetritium exposure device was evacuated overnight to avoid the con-tamination of ambient air when the sample was taken out from theexposure device.

After the evacuation, the sample was taken out and then it wasplaced in front of an ultra-low energy X-ray detector to measure energyspectrum of X-rays induced by β-rays emitted from tritium retained in asample. This measurement technique is called as BIXS (β-ray induced X-ray spectrometry) [11]. Major strong point of this technique is able toevaluate non-destructively the amount of tritium trapped in surfacelayers of a tritium-exposed sample. In addition to this feature, estima-tion of a tritium depth profile in bulk is also possible by analyzing theobserved X-ray spectrum. The surface layers in this paper are defined asa maximum range of β-rays in SS316L, which was estimated to be about0.2 µm in depth. During a measurement, the space between X-ray de-tector and sample was filled with argon as a working gas and both ofthem were shielded by lead blocks to reduce influence of natural ra-diations.

To examine the effects of pre-heat treatment in vacuum on thechemical states and composition of constituent elements of the surfacelayers, surface analysis of a bare SS316L sample was independentlyconducted by applying X-ray Photoelectron Spectroscopy (XPS), whichwas provided in Shizuoka University. The XPS device was installed inthe ultra-high vacuum chamber, which was directly connected to thevacuum chamber for heating a bare SS316L sample. It is possible tomove a sample between both chambers without air exposure. Four bareSS316L samples were employed to analyze after degassing at roomtemperature (RT), 473, 573, and 673 K. The energy steps in the mea-surement of XPS were 0.1 eV, and binding energy of the photoelectronpeaks observed was calibrated by referring Cu2p3/2 as 932.6 eV [12].

Results and discussion

Retention behavior of tritium in bare SS316L samples

Temperature dependence of tritium exposure was examined in theexperiment No. 1. Examples of X-ray spectra observed under the giventritium exposure temperature are shown in Figs. 2[A] and [B]. Plural X-ray peaks were observed for both spectra although the peak intensitieswere quite different: namely, characteristic X-ray peaks attributed to anargon atom and the constituent atoms of SS316L, and bremsstrahlungX-rays of a broad peak (1–8 keV). These X-ray peaks are produced whena β-particle plows through the nearby atoms. That is, a part of β-rayenergy is converted to the characteristic X-rays of Ar(Kα) and Ar(Kβ)(hereafter described as Ar(Kα,β)) in the argon atmosphere, when kineticenergy of β-rays escaped from the surface layers is larger than the en-ergy of K-shell absorption edge (3.2029 keV) of an argon atom. It isknown that X-ray intensity of Ar(Kα,β) is proportional to the amount oftritium retained in surface layers [13]. In the same manner, char-acteristic X-rays from constituent atoms of SS316L should be also at-tributable to the interactions between β-rays and constituent atoms ofSS316L, when β-rays are emitted in the bulk of a sample. Therefore, X-

Fig. 1. Schematic of experimental procedure. Encircled numbers describe theexperimental number.

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ray intensity of Ar(Kα,β) is strongly affected by the amount of tritiumretained in surface layers thinner than 0.2 µm, while the intensity andshape of bremsstrahlung X-ray peak largely depends on a tritium depthprofile in the bulk. All the X-ray peaks observed in Fig. 2[A] are sig-nificantly lower than that in Fig. 2[B], indicating that the tritiumconcentration in the sample exposed at 393 K is quite lower than thatexposed at 623 K. It is clearly seen that the difference in both exposuretemperature conditions is reflected to the X-ray intensity.

Dependence of the degassing temperature prior to tritium exposurewas examined in the experiment No. 2. Correlation between the de-gassing temperature and Ar(Kα,β) peak intensity observed is shown inFig. 3. Significant increase in the intensity of Ar(Kα,β) peak appeared ina temperature region higher than 600 K. This indicates that the tritiumamount in surface layers increased with an increase in the degassingtemperature and drastic change in the tritium retention brought aboutaround 600 K. Examples of the observed X-ray spectra are shown inFigs. 4[A] and [B]. It was seen from the Fig. 4[A] that the amount oftritium retained in the sample became very small when both tempera-tures of degassing and tritium exposure were low. The intensity of Ar(Kα,β) peak was 0.077 counts per min (cpm). By comparing bothspectra, it can be understood that degassing at a high temperaturecauses an great increase in the intensity of Ar(Kα,β) peak even if tritiumexposure temperature is as low as 393 K. In addition to this, a brems-strahlung peak was also observed in Fig. 4[B] with plural characteristicX-ray peaks, indicating that a part of tritium adsorbed on the surfacediffused into the bulk even low temperature. That is, it was suggestedthat significant changes in surface properties of a bare SS316L sample

was caused by degassing at a high temperature and that the tritiumretention was strongly influenced by changes in the surface properties.

Furthermore, to examine the effects of degassing at higher tem-peratures, a preliminarily degassed SS316L sample was temporarilyexposed to an ambient atmosphere at room temperature, and then it

Fig. 2. X-ray spectra observed for bare SS316L exposed to tritium gas at a given temperature. A: 393 K, B: 623 K. The degassing temperature prior to both tritiumexposure experiments was 673 K.

Fig. 3. Dependence of the degassing temperature for bare SS316L sample.Tritium exposure temperature was kept constant at 393 K.

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was degassed at 393 K. Just after this processing, it was exposed totritium gas at the same temperature. The X-ray spectrum observed isshown in Fig. 4[C]. The observed intensity of Ar(Kα,β) peak was almostsame as that shown in Fig. 4[B] despite air exposure and degassing atlow temperature, but no clear bremsstrahlung X-ray peak was observedand intensities of Cr(Kα) and Fe(Kα) peaks became small. These resultsindicate that diffusion of tritium into the bulk was disturbed by changesin surface states because of air exposure. However, it is seen that theamount of tritium retained in surface layers significantly increased incomparison with Fig. 4[A] of which the degassing and tritium exposuretemperature was the same as that in Fig. 4[C]. It is suggested, therefore,that the surface formed by the air exposure is different from that of thebare SS316. The increase in surface tritium may be due to increase inthe trapping sites such as Me-O and/or Me-OH (Me=metallic ele-ments) formed by air exposure.

Retention behavior of tritium in the plasma-exposed sample

Color change from metallic luster to pale dark-indigo was observedfor the plasma-facing surface. Original surface of a bare SS316L samplehas been lost after plasma exposure. This should be due to the de-position of plasma-facing materials and/or oxidation reaction with theoxygen containing species in plasma. The X-ray spectrum observed aftertritium exposure is shown in Fig. 5[A]. The observed intensity of Ar(Kα,β) peak was estimated to be 0.058 cpm. It was 7.5 times larger thanthat observed in Fig. 4[A] when the concentration of tritium gas usedfor tritium exposure was taken into account. On the other hand, nobremsstrahlung X-ray peak was observed. Such a phenomenon was

similar to the observation in Fig. 4[A]. Namely, it was suggested fromthese observations that the surface modification by plasma exposureincreased in the adsorption of tritium but disturbed diffusion of tritium.

Subsequently, this sample was repeatedly used to examine at hightemperature conditions. The sample was degassed stepwise at 473, 573and 673 K in TED-2 and it was exposed to tritium gas at respectivetemperatures. Fig. 5[B] shows the X-ray spectrum observed under thehighest temperature conditions. As clearly seen from the X-ray spec-trum, plural characteristic X-ray peaks and a broad and weak brems-strahlung X-ray peak were observed in addition to the Ar(Kα,β) peak.The former X-ray peaks were assigned to the major constituent atoms ofSS316L. No additional metallic atoms such as W (Mα = 1.774 keV),which was partly used in QUEST as a plasma-facing material, wereobserved in the X-ray spectrum. It was suggested, therefore, that me-tallic atoms in the surface layers basically consist of only constituentelements of SS316L. In addition, as shown in the inset of Fig. 5[B],appearance of a clear bremsstrahlung X-ray peak suggests that thesurface layers modified by plasma exposure is very thin and that dif-fusion of tritium into the bulk can thoroughly take place at 673 K.

Intensity of the Ar(Kα,β) peak observed in Fig. 5[B] largely increasedmore than 170 times in comparison with the result of Fig. 5[A]. Thisindicates that the tritium retention largely increased with an increase inthe temperature even if the tritium concentration was taken into ac-count. Tritium molecules can dissociate to atomic state on metallicsurface at a high temperature and subsequently the atoms easily diffuseinto the bulk. Such diffusion behavior of tritium atoms can be under-stood by appearance of a clear bremsstrahlung X-ray peak though theintensity was not so strong.

Fig. 4. Effects of air exposure after degassing at 673 K. A: degassing temperature was 393 K, B: degassing temperature was 673 K, C: air exposure at room temperatureafter degassing at 673 K and subsequently degassed at 393 K. Tritium exposure temperature in all experiments was kept constant at 393 K.

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Fig. 6 shows the intensity changes in all characteristic X-ray peaksobserved for plasma-exposed SS316L. In this experiment, the tritiumexposure was conducted at the same temperature just after degassingprocess. The intensity of the Ar(Kα,β) peak increased with an increase inthe degassing and tritium exposure temperature, and a drastic increase

was observed after degassing at 573 K, indicating that the plasma-ex-posed surface has significantly changed by degassing at above 573 K.Other characteristic X-ray peaks such as Cr(Kα), Fe(Kα) and Ni(Kα) alsoshowed similar changes with temperature. It was clarified, therefore,that effects of the surface modification by plasma exposure fairly

Fig. 5. X-ray spectra observed for plasma-exposed SS316L. Temperature of the degassing and tritium exposure was the same. A: 393 K, B: 673 K. Inset in the B is anenlarged view.

Fig. 6. Correlation between tritium exposure temperature and peak intensity of characteristic X-rays in the plasma-exposed sample. The degassing temperature wasthe same as tritium exposure temperature.

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disappeared by degassing at a high temperature and that the activesurfaces were again formed.

Surface analysis of bare SS316L samples by XPS

It was suggested from the tritium exposure experiments that thechemical state of surface elements plays an important role for tritiumretention behavior in SS316L. For this reason, changes in the chemicalstates of metallic species on the surface with degassing temperaturehave been examined using XPS. It was seen that the outermost surfaceof all samples was largely covered with the chemical species includingcarbon and oxygen atoms irrespective of degassing temperature, al-though carbon species relatively decreased with increasing temperatureand contrarily oxygen species increased with temperature. It was seenthat the surface composition of SS316L changed with heat processingonly in vacuum. Information of chemical states of metallic species nearsurface will give important knowledge for understanding of tritiumretention behavior.

Changes in the spectra of chromium, iron and nickel with degassingtemperature are described in Figs. 7, 8 and 9, respectively. Chemicalstate of chromium at low temperatures was basically ascribed tochromium oxide (Cr2O3) from binding energy as shown in Fig. 7. Clearappearance of metallic chromium was confirmed when the sample wasdegassed at 673 K. Such a phenomenon was similar to changes in thespectrum of iron which is shown in Fig. 8. Above 573 K, metallic ironappeared. As shown in Fig. 9, existence of metallic nickel was con-firmed even at room temperature, but the peak became obscure withincrease in degassing temperature. Such a change was quite different

from chromium and iron, although the reason is not clear yet. Suchchanges in the chemical states of metallic species near surface wereconsistent with that in tritium retention behavior mentioned pre-viously. Namely, it is considered that degassing at higher temperaturesassists appearance of metallic species. It is concluded, therefore, thatthe tritium retention behavior is strongly affected by history of thethermal and vacuum conditions of PFMs in the fusion devices.

Summary

It is one of great important issues to reduce tritium inventory in theplasma-facing materials. From this viewpoint, the effects of heatingtemperature on tritium retention have been studied using stainless steeltype 316 L (SS316L) samples and a plasma-exposed SS316L sample as amodel. The former samples were as-received SS316L (bare SS316L) andthe latter one was prepared using the plasma experiment device,QUEST. Surface analysis of SS316L samples was separately conductedto get information about changes in the chemical states of surfaceelements by heating processing in vacuum. After exposing to tritium gasunder the given temperature condition, tritium amount retained in thesample was evaluated using a technique of β-ray-induced X-ray spec-trometry.

The amount of tritium retained in a bare SS316L sample showedclear temperature dependence, and it increased with increasing thedegassing temperature and the exposure temperature of tritium. Inaddition, drastic increase appeared above about 600 K as a function ofthe degassing temperature. In particular, the degassing procedure of abare SS316L sample prior to tritium exposure led to a great effect for

Fig. 7. Photoelectron spectra of Cr2p as a function of degassing temperature. Every spectrum was measured without sputtering by argon ions after degassing at eachtemperature. Peak energies of Cr are 574.1 and 583.4 eV, and those of Cr2O3 are 576.6 and 586.5 eV.

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tritium retention behavior. That is, it was seen that tritium retentionincreased even if the tritium exposure temperature was low, when thesample has been once degassed at the high temperature. Similarly,tritium retention in plasma-exposed SS316L increased with an increasewith the degassing temperature, and the effect of heating temperaturedrastically appeared above 573 K. Similar retention behavior was ob-served for plasma-exposed SS316L despite color change of the surface.

Results of surface analysis indicated that metallic states of con-stituent elements of SS316L appeared in the degassing temperatureabove around 573 K though the initial surface was almost covered withcarbon and oxygen species. This was agreed with the temperature thattritium retention was drastically changed. Namely, it is considered thatthe degassing temperature plays an important role for tritium retentionbehavior. It is concluded, therefore, that the tritium retention inplasma-facing materials is strongly affected by history of the thermaland vacuum conditions.

Acknowledgments

This work has been performed with the support and under theauspices of the National Institute for Fusion Science (NIFS)Collaboration Research Program(NIFS12KUTR081). The authors wouldlike to thank Dr. Y. Oya of Shizuoka University for surface analyses ofSS316L samples by X-ray photoelectron spectroscopy and for his valu-able suggestions.

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